1. Field of the Invention
The present invention relates generally to the field of magnetic disk drives, and more particularly to a magnetic disk drive to which a perpendicular magnetic recording method is applied.
2. Description of the Related Art
In recent years, in the filed of magnetic disk drives as typified by hard disk drives, attention has focused on the perpendicular magnetic recording method as a technique for exceeding the recoding density limits imposed in the longitudinal magnetic recording method.
In the perpendicular magnetic recording method, if digital data (0/1) is recorded on a disk medium, a magnetized area corresponding to the data is formed in a perpendicular direction (depth direction) of the disk medium. Further, in the perpendicular magnetic recording method, the amplitude of a read signal read out from the disk by a read head transfers at a magnetization transfer position. The waveform of the read signal is a rectangular wave whose amplitude corresponds to the magnetization direction.
Therefore, if a read channel for the longitudinal magnetic recording method is diverted in a disk drive of the perpendicular magnetic recording method, a differentiation circuit for converting a read signal waveform into a differentiated signal waveform is needed.
In the latest disk drives, a magnetic head having a structure wherein a read head and a write head are separately mounted on a slider is adopted. In general, the write head is a single pole type (SPT) head suitable for the perpendicular magnetic recording method. The read head is a magnetoresistive (MR) head in both the methods, especially, a giant magnetoresistive (GMR) head using a GMR element.
In order to obtain a disk drive achieving a high recording density, improvements in characteristics of the GMR element (including a spin valve type MR element), which is a read head, have been promoted. More specifically, a method for ordering the magnetization directions of a free layer and a pinned layer of the GMR element to cross each other has been adopted. Prior art using the above method to achieve an excellent magnetic domain control of the free layer, thereby reducing Barkhausen noise, etc., has been proposed (refer, for example, to Jpn. Pat. Appln. KOKAI Publication No. 2001-110016).
A method has also been adopted for making the magnetic easy axis directions of the free layer and the pinned layer of the GMR element substantially parallel. Prior art using the above method to significantly vary the magnetization direction of the free layer, with respect even to small amounts of recording magnetic field information, thereby improving the sensitivity to magnetic fields has been proposed (refer, for example, to Jpn. Pat. Appln. KOKAI Publication No. 2001-56908).
Further, prior art has been proposed for providing a structure wherein a soft magnetic layer having a spin valve type MR film comprises a laminated element of two or more layers. Thereby, the detection sensitivity of a magnetic recording signal and the linearity of a read signal are improved, and the output characteristics of a read head are improved (refer, for example, to Jpn. Pat. Appln. KOKAI Publication No. 2000-340858).
In a disk drive of the perpendicular magnetic recording method, in general, a double-layered disk medium (DL disk medium) comprising a perpendicular recording layer and a soft magnetic layer is used. In such a DL disk medium, magnetic recording is carried out, as a data recording operation, by the magnetic coupling between the write head and the soft magnetic layer. Thus, the read head can detect a high recording magnetic field, when recorded data is read out from the disk medium.
On the other hand, in a disk drive of the perpendicular magnetic recording method, output saturation of a GMR element or waveform distortion of a read signal easily occurs, compared with that of the longitudinal magnetic recording method. In order to eliminate these problems, there is provided a method for improving the linearity in the magnetic response characteristic of the GMR element. Specifically, it is a method of increasing the linear response dynamic range to exceed the mean value of the recording magnetic field from the disk medium. Thereby, output saturation of a GMR element or occurrence of waveform distortion of a read signal can be prevented or suppressed.
As described above, in a disk drive of the perpendicular magnetic recording method, the waveform of the read signal output from the read head is a rectangular wave. Thus, the read channel includes a differentiation circuit for converting a read signal waveform into a differentiated signal waveform. The read channel reproduces data recorded on a disk medium from a read differentiated signal output from the differentiation circuit.
The reproduction resolution of the read channel is influenced by the magnitude of the pulse width at 50% threshold (sometimes expressed as PW50) of a read differentiated signal. The PW50 is associated with the degree of nonlinearity in the magnetic response characteristic of the GMR element. It has been recognized that the higher the degree of nonlinearity (i.e., linearity is reduced), the smaller (narrower) the PW50.
In general, in a magnetic recording system, the PW 50 is just as important a parameter as the S/N ratio, regarding the error rate. If the value of the PW50 increases, this results in reduction in reproduction error, especially in a high recoding density. It is desirable that the PW50 value be smaller.